In theoretical physics, top-down cosmology is a proposal to regard the many possible histories of a given event as having real existence.
[1] This idea of multiple histories has been applied to cosmology, in a theoretical interpretation in which the universe has multiple possible cosmologies, and in which reasoning backwards from the current state of the universe to a quantum superposition of possible cosmic histories makes sense.
Stephen Hawking has argued that the principles of quantum mechanics forbid a single cosmic history,[1] and has proposed cosmological theories in which the lack of a past boundary condition naturally leads to multiple histories, called the 'no-boundary proposal', the proposed Hartle–Hawking state.
[2] According to Hawking and Thomas Hertog, "The top-down approach we have described leads to a profoundly different view of cosmology, and the relation between cause and effect.
Top down cosmology is a framework in which one essentially traces the histories backwards, from a spacelike surface at the present time.